In a conventional alphanumeric display, a plurality of individual segments are selectively energized to visually represent a desired character. The most common type of display includes at least seven discrete segments arranged so as to visually represent a numeral eight if all of the segments are simultaneously energized. Although this type of display can represent a limited number of alpha characters, it is most often employed to represent the numbers zero through nine. Substantially the same seven segment configuration (although fabricated very differently for the two types of display technologies) can be used for both light emitting diode (LED) displays and liquid crystal displays (LCDs).
There are several advantages that justify using LCDs rather than LEDs in instrument displays. Particularly important in portable, battery powered instruments is the intrinsically lower power requirement of the liquid crystal technology. In addition, relatively complex arrangements of graphic icons and alphanumeric character displays can readily be configured on a common substrate to produce a complete LCD panel for an instrument. By contrast, LED displays are more directed to characters than graphic icons and not as easily fabricated as complex, integrated panels.
Each segment of an LCD corresponds to similarly shaped electrically conductive regions applied to the front and rear surfaces of the display. These electrically conductive regions are coupled to a control circuit that supplies a voltage appropriate to modify the optical characteristics of a liquid crystal layer disposed between the front and rear surfaces. The electrical signal applied to these regions causes the liquid crystal layer to become more opaque, so that a character or graphic icon corresponding to the shape of the electrically conducting regions is visible. Optionally, background lighting can be provided for an LCD to produce greater contrast so that the characters or icons are more easily visible at low ambient light levels.
There are certain applications in which a failure of one of the segments that defines a character or graphic icon may have life-threatening consequences. For example, a display panel on a medical instrument may indicate certain critical operating parameters to an operator of the instrument. Clearly, in this instance, it is very important to avoid errors in reading the displayed data. An error caused by the failure of a segment in a numeric display character would be particularly serious if the character is the most significant digit of a critical displayed value. For instance, failure of the center segment in a seven segment character of the display would cause a "0" to visually appear as an "8." A medical practitioner relying on the incorrect displayed reading caused by such a failure might use the instrument in a manner that harms a patient. Accordingly, for any critical displayed parameter on a medical instrument, designers have recognized the importance of detecting a display failure so that the user is alerted and does not rely upon an incorrect value.
One way to insure that a failure in a critical display character is detected is to duplicate the entire displayed parameter, so that two nominally identical values for the parameter are indicated in separate displays. If the two displayed values are different, the user is supposed to recognize that a failure has occurred in one of the duplicated displays. However, because the two displayed values are spatially separate, such difference may go unnoticed. Moreover, space limitations on a display panel often render it impractical to provide duplicate values of a parameter, and this solution to the problem is inelegant at best.
If LEDs are used for such critical displayed parameters, the electrical current to each digit of the display can be monitored and compared to an expected value in a look-up table that relates each of the possible characters to the electrical current draw required for that character, based on the number of segments that must be selectively energized to represent it. For example, a numeral "1" requires that only two segments be energized (in a seven segment display), and the look-up table defines a current corresponding to that required by the two segments. If less current is detected, at least one of the two segments may have failed and a monitoring circuit alerts the operator with a visual and/or audible alarm. Similarly, if current to a segment is detected when that segment should not be energized, the monitoring circuit also detects a failure.
Unfortunately, the low current requirements of LCDs make it practically impossible to detect a failed segment in an LCD character based on electrical current measurements. Consequently, although LCDs are almost uniformly preferred for display panels because of the variety of graphic options that can be included and because of their low power requirement, LEDs are often used for the display of critical parameter values, simply to ensure that the failure of a segment in the critical display can be detected by the current monitoring method. Consequently, both an LCD display panel and LEDs must be used on such instruments. It should be evident that it would be preferable to use only LCDs in a display panel, if failure of a segment of any LCD characters was clearly evident, especially if such a failure did not preclude the correct character from being visually perceived.